Quantitative analysis of complex mixtures using mass spectrometry is one of the most important territories in analytical chemistry. Each phase of the quantitative analysis procedure should be carefully optimized and precisely calibrated. Representative methods for quantitative analysis includes high-performance liquid chromatography-mass spectrometry (HPLC-MS) and gas chromatography-mass spectrometry (GC-MS). Both have been developed for decades and are already widely used for drug metabolism, biomarker discovery, protein/lipid study, environmental monitoring, food safety and forensic applications. A general procedure using a modern MS analysis system typically starts from sample preparation. Sample preparation typically involves analytes being concentrated, purified and extracted into pure solution, then chromatographically separated and analyzed using mass spectrometry in a successive manner. Either external standards or internal standards should be used for calibration. LC/GC-MS is a powerful method for quantitative analysis for complex mixtures, but is still labor intensive and time consuming, the operators also must be highly trained to use the instrument and to design and troubleshoot methods.
Elimination of sample preparation benefits MS analysis in quantitation, which has become a reality by using ambient ionization methods. Ambient analysis involves the interrogation of samples in their native environment to reduce the time required for analysis and to simplify the operations. One drawback is that analysis of untreated complex samples can lead to ion suppression, in which the detection of the analyte of interest is compromised due to the presence of other interfering chemicals. The sensitivity and reproducibility of the method can suffer due to these matrix effects. As in traditional quantitative MS analysis, the introduction of internal standards allows the best quantitative performance for ambient analysis. A problem with methods for introducing internal standards to samples is accurate reproducibility when dealing with small volumes. In such circumstance, using an air displacement pipette to transport sub-microliter liquid may result in a 12% error in pipetting accuracy, which is even worse for blood. Spiking internal standard into samples in a vial using pipetting and vortex mixing is not suitable either when dealing with microliter samples.